1. The Field of the Invention
The present invention is directed to color filters and methods for preparing color filters. In particular, the present invention is directed to color filters for use in visual displays and methods for preparing the same.
2. The Relevant Technology
Color filters are used to produce full color images in visual displays. The three primary colors, red, green and blue used to produce full color images in projection displays, flat panel displays and other visual display devices are most commonly provided by color filters.
Generally, color filters consist of a transparent substrate having a repeating pattern of pixels on its surface. Pixels are defined as the smallest controllable area on visual displays, having the same color that are capable of being located and turned xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d by a computer display. Each pixel on a color filter is associated with a primary color, and is arranged with other color pixels in repeating arrays of red, green and blue triads. Depending upon which pixels light is passed through, color filters containing pixels of the three primary colors are capable of producing color images of a wide variety of colors.
Although the use of color filters to produce full color images has long been known, color filters continue to occupy the highest proportion of material costs in visual displays, such as flat panel displays. This expense can be attributed to a number of factors, such as color filter material costs and the number of manufacturing steps; however, the high cost of color filters is primarily due to low yields observed in color filter manufacturing processes.
Yields in color filter manufacturing process are determined by the number of color filters produced that meet industry standards out of the total number of filters produced. Panels not meeting industry standards of approximately 3 to 4 defects per ten inch diagonal must be discarded thereby reducing color filter manufacturing yields and resulting in high production cost and low consumer availability. A common defect and major contributor to the low yields presently experienced in the industry is the omission of pixels, a phenomenon commonly referred to as xe2x80x9cdrop out.xe2x80x9d When xe2x80x9cdrop outxe2x80x9d occurs, uncolored, unfiltered light passes through the filter to the eye of an observer as opposed to the color intended to be produced.
Originally, color filters were prepared by a dyeing, or gelatin process in which a layer of gelatin or other dyeable material formed on the interior of a transparent substrate was colored using photolithography techniques. More recently, polyimide systems comprising thermally stable dyes combined with polyimides, have been incorporated into photolithography processes to improve filter quality. Although color filters prepared using photolithography exhibit good resolution and color quality, photolithography is labor intensive and results in poor yields. For example, photolithography requires that each color incorporated onto the filter have a mask, a photoresist, baking and etching steps, and resist removal. To produce a color filter having the three primary colors, this process must be repeated three times.
Because of the complexity of the photolithography process and the tedious steps that must be repeated for each primary color, photolithography processes have consistently given unsatisfactory yields, typically on the order of 50%. Even after repairing defective color filters by repeating the photolithography process for omitted pixels, the yield only increases to approximately 70%. Taking into consideration the cost of repair and the high percentage of filters that remain unusable, there has for some time existed a need for a color filter manufacturing process having greater yields and consequently producing color filters at lower costs.
More recently, in an attempt to overcome the above-mentioned deficiencies, a dye diffusion process for making color filters has been proposed. In this process, a sublimable dye is transferred from a donor sheet containing the dye to a polymeric receiver sheet which becomes the color filter. An exemplary dye diffusion process is disclosed in DeBoer et al. U.S. Pat. No. 4,965,242. In DeBoer et al., a dye-donor element is placed over a dye receiving element, wherein the dye receiving element comprises a temporary support having thereon a polymeric alignment layer, transparent conducting layer and a polymeric dye receiving layer. Heat is applied to the donor element by radiation energy means, such as a thermal printing head or heat absorption by infrared dyes, causing the dye image to be transferred from the donor sheet to the receiver sheet. Once the transfer has occurred, the dye donor sheet is replaced with a glass support to form the color filter. Other patents disclosing similar dye diffusion processes for preparing color filters include U.S. Pat. Nos. 4,962,081, 5,073,534, and 5,242,889.
Here again, although the dye diffusion process simplifies color filter manufacturing, adequate yields are still not attained. An additional drawback that adds to the low yields and increased cost of the color filter is that the dye diffusion process is presently limited to sublimable subtractive dyes, namely magenta, yellow and cyan.
Producing primary colors using subtractive dyes requires layers of magenta, yellow and cyan to be formed in various combinations to make red, blue and green additive colors. One exception is found in U.S. Pat. No. 5,242,889, issued to Shuttleworth, which discloses a blue sublimable dye. However, because the dye diffusion process must be repeated to form pixels associated with red and green, there exists a greater opportunity for a defect to occur, resulting in an increased number of unusable color filters and, consequently, the continuation of depressed yields.
As expected, the xe2x80x9cdrop outxe2x80x9d rate in color filter manufacturing increases during mass production. In addition, as illustrated in FIG. 1, as the panel size increases, the number of xe2x80x9cdrop outsxe2x80x9d rises dramatically. For example, a defect level of one defect per square inch sharply increases to approximately 50 defects per ten inch diagonal color filter as shown by line 30. A marked decrease can be attained by limiting the defects to 0.1 defects per square inch (line 32) and as shown by lines 34 and 36, the number of defects drops sharply when the number of defects is reduced to {fraction (1/16)} of a defect per square inch (line 34) and {fraction (1/160)} of a defect per square inch (line 36). Typically, color filters have dimensions of approximately 200 pixels per/inch (wherein 5 mils is equivalent to 127 microns). Panels having as few as three to four defects per ten square inch diagonal panel are considered unusable according to industry standards. Hence, it is not surprising that typical yields of current color filter manufacturing processes have heretofore been on the order of 50% and with repair approximately 70%.
Because of its high color quality, and despite attempts to improve its low yields, photolithography remains the process of choice in the production of color filters for visual displays. This being the case, there remains a need for a manufacturing process that produces color filters having good resolution and color quality in high yields.
It is, therefore, an object of the present invention to provide color filters for use in visual displays that are lower in cost than have heretofore been available.
It is another object of the present invention to provide methods for making color filters for visual displays that result in higher yields than have heretofore been attained.
It is a further object of the present invention to provide a color filter having good color quality and high resolution.
To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein, the present invention is directed to color filters comprising a transparent substrate and a repeating pattern of colored pixels on the substrate thereby forming the color filter, wherein each of the pixels comprises a plurality of small colorant areas, hereinafter referred to as sub-pixels.
In accord with the present invention, and contrary to conventional knowledge, it has been discovered that processes forming pixels comprising a plurality of sub-pixels exhibit a drastic increase in the formation of usable color filters. When pixels comprising a plurality of sub-pixels are formed, the omission of less than all of the plurality of sub-pixels comprising the pixels will not necessarily result in an unusable pixel. Thus, color filters formed in accordance with the present invention have greatly improved yields. The repeating pattern of pixels are composed of a colorant material selected from the group consisting of dyes, pigments, inks, and mixtures thereof. Depending on the process employed, the materials used, and the purpose of the color filter, the colorant material used to form the pixels is selected so as to optimize the color quality of the color filter.
Resolution of the color filter formed in accordance with the present invention, is optimized by varying the size of the pixels and the number of sub-pixels comprising the pixel. For instance, reducing the size of the pixels in the repeating pattern of pixels and increasing the number of pixels on a transparent substrate produces a color filter having increased resolution.
In a preferred embodiment of the present invention, the color filter comprises a transparent glass substrate and a repeating pattern of color pixels on the substrate forming the color filter, wherein each pixel is comprised of sixteen sub-pixels. With each pixel comprising sixteen sub-pixels, the omission of a few sub-pixels constructing the pixel is not visible to the human eye. Furthermore, it is highly improbable that more than three or four sub-pixels will be omitted during pixel formation. Hence, virtually one hundred percent yield will be achieved when the pixels formed on the substrate are composed of 16 sub-pixels. Higher than current yields may also be obtained when using pixels having more than one but fewer than 16 sub-pixels, although the yield may not be as high as when using 16 sub-pixels.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.